A variety of impregnated catalysts have been practically applied in the polymerization of olefins such as ethylene and propylene to prepare polyolefin. Among them, a metallocene catalyst, which is prepared by impregnating metallocene compound on inorganic carriers such as alumina and silica for the preparation of polyolefin, has prevailed in the art.
In general, a metallocene catalyst is prepared by adding an organic solvent and an organic aluminum compound to inorganic carriers such as alumina and silica under an environment of inert gas, to modify the surface of the inorganic carrier, and impregnating the surface-modified inorganic carrier with the metallocene compound, and drying.
On the other hand, polyolefin is prepared by adding the metallocene catalyst thus prepared to organic solvent and one or more olefin monomers such as ethylene, carrying out slurry or liquid type polymerization under a constant temperature and pressure in the presence of excessive amounts of organic aluminum compound such as methylaluminoxane as a cocatalyst, adding hydrogen to the reaction mixture in order to modulate molecular weight and terminating the reaction.
Since the supported metallocene catalyst plays a critical role on the properties of polyolefin such as tacticity, shape of polymer particle, molecular weight, and distribution of molecular weight, etc., studies on the preparation of metallocene catalysts having more improved properties have been actively carried out.
For example, U.S. Pat. No. 4,808,561, U.S. Pat. No. 4,912,875 and U.S. Pat. No. 4,904,631 teach a process for preparing a metallocene catalyst by modifying the surface of an inorganic silica carrier with methylaluminoxane or modifying the surface of water-deposited silica with alkyl aluminum and impregnating with metallocene compounds such as bis(cyclopentadienyl) zirconium dichloride, etc.
Also, U.S. Pat. No. 5,308,811 describes a process for preparing a metallocene catalyst by treating the surface of alumina with methylaluminoxane and impregnating with metallocene compound.
However, these prior art metallocene catalysts are proven to be less satisfactory in the sense that: polyolefin can not be produced with a high yield, since the conventional metallocene catalyst has low activity; shaping of the polymer is not so good, since shape of polyolefin thus prepared is not spherical; properties of polyolefin are not good, since a lot of remnants such as aluminum and metallocene metal remain within the prepared polymer which is caused by the excessive use of cocatalyst; and, the cost for polyolefin preparation is extremely high.
On the other hand, EP 436,326 A1 and EP 435,514 A2 disclose a process for preparing a metallocene catalyst in which prepolymerizing reaction was carried out by impregnating the metallocene compound and an electron donor compound on an inorganic carrier of magnesium chloride. The metallocene catalyst thus prepared permits preparation of polyolefin possessing reduced amount of remnants such as metallocene metal, etc. However, the prior art, like the above-mentioned prior art, has revealed some shortcomings: polyolefin can not be produced with a high yield, since the metallocene catalyst has low activity; and, shaping of the polymer is not so good, since the shape of polyolefin thus prepared is not spherical.
Under the circumstances, the present inventors have found that the problems of conventional supported metallocene catalysts result from the use of amorphorous or paracrystalline inorganic carriers such as silica having irregular pores whose spatial arrangement is distributed widely.